Unaudited Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net income	$ 64,616	$ 63,015
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	14,916	12,215
Accretion of discount on available-for-sale securities	(77)	(467)
Loss on disposition of property and equipment	132
Amortization of debt issuance costs	9	9
Stock-based compensation expense	23,581	15,811
Cash paid for derivative settlement	(232)	(69)
(Gain)/loss on derivative	(424)	1,644
Deferred income taxes, net	2,738	1,392
Changes in operating assets and liabilities:
Accounts receivable	(10,662)	1,239
Prepaid expenses	(6,255)	(4,292)
Inventory	(125)	254
Other assets	559	(1,801)
Deferred contract costs	(26,575)	(27,630)
Accounts payable	803	408
Income taxes, net	23,424	19,973
Accrued commissions and bonuses	(6,922)	(6,681)
Accrued payroll and vacation	8,105	6,289
Deferred revenue	2,167	2,211
Accrued expenses and other current liabilities	(321)	(1,490)
Net cash provided by operating activities	89,457	82,030
Cash flows from investing activities
Purchase of short-term investments from funds held for clients	(47,215)	(177,903)
Proceeds from maturities of short-term investments from funds held for clients	80,000	20,000
Purchases of property and equipment	(25,330)	(25,726)
Net cash provided by (used in) investing activities	7,455	(183,629)
Cash flows from financing activities
Repurchases of common stock		(7,998)
Withholding taxes paid related to net share settlements	(377)	(170)
Payments on long-term debt	(444)	(444)
Net change in client funds obligation	686,190	(270,399)
Net cash provided by (used in) financing activities	685,369	(279,011)
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents	782,281	(380,610)
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period	1,585,275	1,641,854
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period	2,367,556	1,261,244
Cash and cash equivalents	215,093	181,827
Restricted cash included in funds held for clients	2,152,463	1,079,417
Non-cash investing and financing activities:
Purchases of property and equipment, accrued but not paid	6,217	8,251
Stock-based compensation for capitalized software	1,456	1,601
Right of use assets obtained in exchange for operating lease liabilities	$ 541	$ 3,536